1. Field of the Invention
The present invention relates to a receiving circuit, and more particularly to a receiving circuit which permits simplification of its circuit arrangement and which further allows less power for a receiving system concurrent with reduction of power consumption.
2. Description of the Prior Art
One of important points needed for a receiving circuit in communications is to possibly reduce high-frequency circuit portions to thereby lessen a high power consumption factor, unstable operation factor and high manufacturing cost inherent in the high-frequency circuit, coupled with a space occupied by that circuit. These points are particularly important for mobile communication systems and others. For the reduction of the high-frequency circuit portions, a direct demodulation method has been proposed heretofore in terms of multiple frequency conversion and carrier frequency, thus accomplishing the direct conversion into a low frequency and the direct demodulation into a baseband. The aforesaid high-frequency circuit portion mainly constitutes a space diversity reception function necessitating two antenna systems.
As the direct demodulation method, there have been developed a number of methods in which a local oscillator generates a signal with a frequency equal to the carrier frequency which in turn, is mixed with a received input wave to derive a baseband signal therefrom. This direct demodulation method is made to produce a high-frequency signal with a frequency equal to the received signal frequency, and the high-frequency signal can be easy to release or emit in the air through an antenna of the receiver. Accordingly, another receiver adjacent thereto undergoes interferences to be inhibited from establishing communications. For this reason, this method has chiefly been adopted for communications based on frequency modulation methods which are relatively strong to single frequency interferences.
On the other hand, radio portable telephones, being recently rapidly put in widespread use, rely on a so-called PSK which is one amplitude transport modulation method, and the single frequency interference produces an offset in a demodulated output to deteriorate the error rate of a received signal. That is, since the local oscillation frequency can not take the carrier frequency, in this kind of communication method difficulty is encountered in direct frequency conversion and direct demodulation. One approach to resolve such a technical problem involves a method which, if a carrier frequency for a radio portable telephone is taken as fc and an offset frequency is taken to be fo, obtains fc+fo and fc-fo in order to provide a frequency-offset complementary local oscillation frequency for a frequency conversion. For carrying out this method, fc+fo and fc-fo are obtainable by multiplication process of fc and fo through a mixer (frequency mixer), while both signals fc+fo and fc-fo coexist in an output. More specifically, although the aforesaid process independently requires signals with the respective frequencies, the prior system can not practically satisfy this requirement. The prior system essentially employs filters for the respective frequencies, while suffering a disadvantage that a carrier frequency for a desired signal is variable and hence the filters need to designed to cope with the frequency variations.